J. Indian Chem. Soc., REVIEW Vol. 83, August 2006, pp. 762-776 Electron transfer reactions between chloroaurate(III) complexes and various inorganic and organic reducing agents in acid medium - a brief review Biswajit Pala*, Pratik K. Senb and Kalyan K. Sen Guptab* aDepartment of Chemistry. St. Paul's C. M. College, 33/1. Raja Rammohan Roy Sarani, Kolkata-700 009, India E-mail : palbiswajit@yahoo.com bDepartment of Chemistry, Jadavpur University, Kolkata-700 032-, India Manuscript received 9 December 2005, revised 2 May 2006, accepted 12 May 2006 Abstract : The review narrates the electron transfer reactions of some chloroaurate(lll) complexes in acid medium re- ported since 1970 onwards. The reactions involving inorganic ·substrates and gold(m) were studied mostly in HCI me- dium whereas the oxidations of different organic substrates were investigated in sodium acetate-acetic acid buffer me- dium. Increase in [H+I decreases the rate of reactions. At higher concentrations of [H+] (ca. 2: 0.5 mol dm- 3 ), AuCJ 4 - or IIAuCJ 4 is the reactive gold(m) species whereas when the reactions were studied in much lower acidities of [H+] ( << 0.5 mol dm- 3 ) or in sodium acetate-acetic acid buffer medium, AuCI 4 -, AuCI 3 (0H 2 ) and AuCJ 3 (0H)- are the re- active species which oxidize the substrates. Kinetic evidence for the participation of AuCI 4 -, AuCI 3 (0H 2 ) and AuCJ 3 (0H)- is provided. The reactivity of these different gold(m) species follow the order : AuCJ 3 (0H)- > AuCI 3 (0H 2 ) > AuCJ 4 -. Influence of ionic strength was noticed in the oxidations of some inorganic and organic compounds by gold(m). The rates in the presence of different salts like NaCI, NaCI0 4 and Na 2 S0 4 at constant ionic strength was found to be same in the oxidations of a number of inorganic compounds indicating that anions do not have any inlluence on the rate. The rates were found to depend not only on the ionic strength hut also on the concentration and nature of the cations. The pseudo-first-order rate constant increases in the order u+ < Na+ < < Rb+ < cs+ in the oxidation of a number of inorganic substrates. The reactions exhibit diverse mechanistic behavior. Kinetic and mechanistic features associated with such reactions have been discussed and analyzed. An attempt has been made to correlate mainly our experimental observations. Keywords : Electron transfer reactions, auric complexes, reducing agents. Introduction Gold is widely distributed in nature and the chemistry of gold remains an active research area 1 <a>. Gold is not es- sential for any Ii\·ing organism though some plants con- centrate the element 1 <b>. Gold is known to exist in differ- ent oxidation states such as + 1, +2, +3, ( +4) and ( +5) for which the electronic configurations are 4 10 , cfJ, /', (d 7 ) and respectively. Of these different states, the underlined states are the most important and the states in the parenthesis are unconfirmed. Complexes of different oxidation states of gold have been discussed and incorpo- rated by Puddephatt 2 and other workers 3 . Gold(III) forms a number of stable complex ions. The anionic complexes 4 like AuF4, AuCI4, Au(CN)4 and Au(N0 3 )4 and four coordinated cationic complexes 5 such as [AuCI 2 py 2 ]Cl, [Au(phenhC1 2 ]CI and [Au(bipyhCI 2 ]CI have been reported in the literature. 762 The lowest and highest coordination number found in coordination compounds of gold are two and six respec- tively with the intermediate number four being the most frequent for gold(m) and two for gold(I) compounds. A wider range of coordination numbers (two to twelve) ex- ists in organometallic cluster compounds. Some gold(I) compounds are biologically active and used as anti inflammatory drugs in the treatment of rheu- matoid arthritis 6 -9. Several gold(III) compounds were also used earlier in the treatment of tuberculosis 10 whereas a few gold(III) complexes are found to have antitumour activity 11 • 12 • Again such gold compounds are found to have severe toxic effects 13 • 14 on human health such as kidney damage and blood disorders. Such toxic effects are presumed to be related to gold(I)-gold(III) oxidation in vivo followed by gold(III)-biomolecule interactions IS( a). Redox and ligand exchange reactions of potential gold(I) and gold(III)-cyanide metabolites under biomimetic con-